1. Field of the Invention
The present invention relates to a relief valve used in a hydraulic control system, in particular in a hydraulic system of a power steering apparatus for a vehicle.
2. Description of the Related Art
As a general structure of a conventional relief valve, there is a structure disclosed in Japanese Utility Model Application No. 56-23765 (reference document 1). This conventional relief valve is provided with a valve housing in which a valve hole is pierced along an axis, and a valve seat member fitted and fixed to an opening end portion of the valve hole. Pressurized oil discharged from a hydraulic pump is introduced to one end portion of a communication hole penetrating along an axis. A valve seat is formed in another end portion, and a ball is brought into contact with the valve seat so as to close the communication hole. A ball receiver holding the ball is slidably supported along an inner surface of the valve hole. A relief spring energizes the ball receiver in a direction in which the ball is brought into contact with the valve seat, and a relief hole relieving the pressure oil at the time of relieving the pressure oil is open in a radial direction of the valve hole. However, in this structure, the ball receiver resonates based on a vibration generated in the vane pump, and this generates a bleep sound.
Accordingly, as a relief valve for preventing the ball receiver from vibrating, there is a relief valve in which a vibration preventing ring constituted by a rubber O-ring and a PTFE ring is interposed between the ball receiver and the relief spring, as described in Japanese Patent No. 2875852 (reference document 2). The vibration preventing ring is brought into contact with the inner surface of the valve hole by pressing the vibration preventing ring against the ball receiver by the relief spring and setting the rubber O-ring of the vibration preventing ring to an expanded state, thereby preventing the ball receiver from vibrating.
In the relief valve described in reference document 2, the rubber O-ring of the vibration preventing ring is compressed in the axial direction between the rubber O-ring and the ball receiver by the relief spring so as to be set in the expanded state in the diametrical direction. The O-ring in the compressed and expanded state generates a sticking phenomenon based on a hardening of the rubber during low oil temperature conditions, and there is a risk that the oil pressure is not relieved even when the oil pressure gets over a set pressure. In this case, a burden is imposed on the other portions. Further, operation of the relief valve is not normally performed due to a change in the set pressure caused by a long time deterioration, and there is generated a phenomenon that oil leakage is generated in the cylinder and the pump.